There are numerous measuring tools equipped with multiple caliper arms which deflect outwardly from a tool body. There are other downhole logging devices which use extendable arms which move independently of one another to position sensors in contact with the side wall of the well borehole which confines such a tool during use. In general terms, the arms must be forced outwardly so that they contact with certainty against the surrounding wall of the borehole to assure that correct and proper measurements are obtained thereby. Ordinarily, the total number of arms is at least two, typically four. It is difficult to mount four arms for pivotal movement. Each arm must have an associated individual spring which provides the loading force applied to the arm to cause rotation. The four arms thus require four springs, and it is difficult to locate four similar springs all within the common body of the caliper tool housing. The housing may be relatively slim, measuring only two to four inches in diameter. This physical constraint makes it difficult to position all the requiste springs in the housing for operation.
The present disclosure sets out a common or single spring system enhanced with a hydraulic coupling system so that each of the deflected arms is driven in similar fashion so that a common force is applied to all arms. The present apparatus thus operates two or more caliper arms which are pivotally mounted on bell cranks with associated push rods. The push rods extend into a hydraulically closed chamber. Each rod serves as a piston. The chamber is filled with hydraulic fluid which is delivered under pressure. As that pressure is increased, the force acting on each push rod is likewise increased. Pressure to the chamber is controllably applied by an external coupling rod which couples to the chamber through a coil spring. The coil spring defines a force which is also applied to the chamber. The chamber is thus loaded to a specified pressure within the chamber and acts on all the push rods within the chamber. This causes the arm to open and permits each rod to move freely and independently during deflection. This utilizes a single spring which reduces the complexity of packaging multiple parallel springs within the housing subject to space limitations. This further makes the chores of assembly and replacement much easier. The latter is especially important when the caliper arms have to be changed to accommodate different dimension wells. Moreover, the hydraulic system set forth herein is substantially free of expensive hydraulic pumps, valves and associated apparatus and thus is a relatively inexpensive tool.